Three specimens of Atractus natans were found during fieldwork in the “Reserva Mamirauá”, Amazonas and a fourth one in the “Estação Científica Ferreira Penna”, Floresta de Caxiuanã, Pará, Brazil. The specimens from Mamirauá were all collected in floating logs in várzea forest during the period of flooding. The new species is small (maximum total length 308 mm) with a coniform head, has 17-17-17 smooth dorsals, without apical pits, male lacking pre-anal keels, ventrals 136-162, caudals 19-25, anal undivided. One or two postoculars, loreal 1.6 –2.8 times as long as high, frontal triangular, wider than long. Maxillary teeth 5-6. The hemipenis is bilobed, with bifurcated sulcus spematicus, capitulum with calyces covered by very small spines, body covered by spines of different size. The back is (dark) brown without a distinct pattern, although on the flanks small darker spots more or less in longitudinal rows are present, a light collar on the posterior part of the head, (indications of) a light area on the snout, throat with black spots sometimes arranged in oblique lines, a wide median black band on the white (in life partly brick red) belly, and underside of tail dark with light spots (brick red in life) at the posterior end of the scales. Palavras-chave: Reptilia; Colubridae; Atractus spec. nov.; Brasil; Amazônia; Mamirauá; Caxiuanã; habitat; várzea.